STOCHASTIC CHARGE TRANSPORT IN MULTI-ISLAND SINGLE-ELECTRON TUNNELING DEVICES

The physics of the operation of singe-electron tunneling devices (SEDs) and singe-electron tunneling transistors (SETs), especially of those with multiple nanometer-sized islands, has remained poorly understood in spite of some intensive experimental and theoretical research. This computational study examines the current-voltage (IV) characteristics of multi-island single-electron devices using a newly developed multi-island transport simulator (MITS) that is based on semi-classical tunneling theory and kinetic Monte Carlo simulation. The dependence of device characteristics on physical device parameters is explored, and the physical mechanisms that lead to the Coulomb blockade (CB) and Coulomb staircase (CS) characteristics are proposed. Simulations using MITS demonstrate that the overall IV characteristics in a device with a random distribution of islands are a result of a complex interplay among those factors that affect the tunneling rates that are fixed a priori (e.g. island sizes, island separations, temperature, gate bias, etc.), and the evolving charge state of the system, which changes as the source-drain bias (VSD) is changed. With increasing VSD, a multi-island device has to overcome multiple discrete energy barriers (up-steps) before it reaches the threshold voltage (Vth). Beyond Vth, current flow is rate-limited by slow junctions, which leads to the CS structures in the IV characteristic. Each step in the CS is characterized by a unique distribution of island charges with an associated distribution of tunneling probabilities. MITS simulation studies done on one-dimensional (1D) disordered chains show that longer chains are better suited for switching applications as Vth increases with increasing chain length. They are also able to retain CS structures at higher temperatures better than shorter chains. In sufficiently disordered 2D systems, we demonstrate that there may exist a dominant conducting path (DCP) for conduction, which makes the 2D device behave as a quasi-1D device. The existence of a DCP is sensitive to the device structure, but is robust with respect to changes in temperature, gate bias, and VSD. A side gate in 1D and 2D systems can effectively control Vth. We argue that devices with smaller island sizes and narrower junctions may be better suited for practical applications, especially at room temperature.

Ermittlung optimaler Bauteilorientierung zur Verbesserung der Prozessplanung in der CAD/RP-Kette

Ein wesentlicher Einflussfaktor auf die Bauteilqualität und Prozessgüte bei der generativen Herstellung von Prototypen ist die Orientierung der Bauteile. So kann eine optimierte Ausrichtung den Treppenstufeneffekt (Staircasing) sowie den Curling-Effekt minimieren und somit die Oberflächenqualität bzw. die Bauteilgenauigkeit erhöhen oder die Berücksichtigung von Formtoleranzen (z.B. Rundheit) ermöglichen. Des Weiteren können verschiedene Bauteilausrichtungen unterschiedliche Ausführungen von Stützkonstruktionen bewirken und die Bauteilstabilität beeinflussen. Diese und ähnliche Wechselwirkungen gilt es bei der Auswahl einer geeigneten Bauteilorientierung für RP-Anwendungen zu berücksichtigen. Dieser Vortrag stellt ein generisches System vor, welches unter Berücksichtigung der genannten Einflussfaktoren sowie weiterer Effekte eine rechnergestützte Optimierung der Bauteilorientierung durchführt. Neben der weiterhin notwendigen Erfahrung der Anwender zur endgültigen Festlegung der fallabhängigen Bauteilausrichtung liefert das System Vorschläge auf Basis einer intensiven Geometrieanalyse, die eine entsprechende Datenaufbereitung im Rahmen der Prozessplanung unterstützen.

K-feldspar hygrochronology

The significance of the multi-isotopic record preserved in K-feldspars is assessed on samples from the Aar metagranite, Central Alps, Switzerland having very tight independent geological constraints. Stepwise leaching reveals that two diachronically grown K-feldspar generations coexist: Kfs-1 (≥ 35 Ma old, Ca-poor, Rb-Cl-rich, with low 87Sr/86Sr and high 206Pb/204Pb) and Kfs-2 (≤ 10 Ma old, antithetic isotopic signatures deriving from external fluids). Microtextures imaged by cathodoluminescence, backscattered electrons, and electron probe microanalysis are patchy and chemically heterogeneous, with pronounced enrichments in Ba in the retrogressed regions. This confirms the simultaneous presence of fluid-dominated retrogression and recrystallization and isotopic inheritance. The staircase-shaped 40Ar/39Ar age spectrum correlates with the Ca/K and Cl/K signatures. This reflects a mixture of heterochemical K-feldspar generations, and not an intracrystalline Ar gradient caused by diffusion. The shape of the age spectrum and the in vacuo release kinetics proceed from entirely different physical and geological phenomena. What K-feldspars can be effectively used for is to constrain the timing of the fluids that interacted with them by multi-isotopic analyses, rather than to model a “cooling history” from 39Ar release alone. The identification of multiple mineral generations by imaging combined with multi-isotopic analysis enables the accurate dating of the events of a multistage evolution after the initial crystallization of the rock in which the minerals occur.

A re-examination of petrogenesis and 40Ar/39Ar systematics in the the Chain of Ponds K-feldspar: "diffusion domain" archetype versus polyphase hygrochronology

K-feldspar (Kfs) from the Chain of Ponds Pluton (CPP) is the archetypal reference material, on which thermochronological modeling of Ar diffusion in discrete “domains” was founded. We re-examine the CPP Kfs using cathodoluminescence and back-scattered electron imaging, transmission electron microscopy, and electron probe microanalysis. 40Ar/39Ar stepwise heating experiments on different sieve fractions, and on handpicked and unpicked aliquots, are compared. Our results reproduce the staircase-shaped age spectrum and the Arrhenius trajectory of the literature sample, confirming that samples collected from the same locality have an identical Ar isotope record. Even the most pristine-looking Kfs from the CPP contains successive generations of secondary, metasomatic/retrograde mineral replacements that post-date magmatic crystallization. These chemically and chronologically distinct phases are responsible for its staircase-shaped age spectra, which are modified by handpicking. While genuine within-grain diffusion gradients are not ruled out by these data, this study demonstrates that the most important control on staircase-shaped age spectra is the simultaneous presence of heterochemical, diachronous post-magmatic mineral growth. At least five distinct mineral species were identified in the Kfs separate, three of which can be traced to external fluids interacting with the CPP in a chemically open system. Sieve fractions have size-shifted Arrhenius trajectories, negating the existence of the smallest “diffusion domains”. Heterochemical phases also play an important role in producing non-linear trajectories. In vacuo degassing rates recovered from Arrhenius plots are neither related to true Fick’s Law diffusion nor to the staircase shape of the age spectra. The CPP Kfs used to define the "diffusion domain" model demonstrates the predominance of metasomatic alteration by hydrothermal fluids and recrystallization in establishing the natural Ar distribution amongst different coexisting phases that gives rise to the staircase-shaped age spectrum. Microbeam imaging of textures is as essential for 40Ar-39Ar hygrochronology as it is for U-Pb geochronology.

Hierarchical self-assembly of nucleotide-appended oligopyrenotides into defined supramolecular objects

Supramolecular DNA assembly blends DNA building blocks with synthetic organic and inorganic molecules giving structural and functional advantages both to the initial self-assembly process and to the final construct. Synthetic molecules can bring a number of additional interactions into DNA nanotechnology. Incorporating extended aromatic molecules as connectors of DNA strands allows folding of these strands through π-π stacking (DNA “foldamers”). In previous work it was shown that short oligopyrenotides (phosphodiester-linked pyrene oligomers) behave as staircase-like foldamers, which cooperatively self-assemble into two-dimensional supramolecular polymers in aqueous medium. Herein, we demonstrate that a 10-mer DNA-sequence modified with 7 pyrene units (see illustration) forms dimensionally-defined supramolecular polymers under thermodynamic conditions in water. We present the self-assembly behavior, morphological studies, and the spectroscopic properties of the investigated DNA-sequences (illustrative AFM picture shown below).

Hierarchical self-assembly of nucleotide-appended oligopyrenotides into defined supramolecular objects

Supramolecular DNA assembly blends DNA building blocks with synthetic organic molecules giving structural and functional advantages. Incorporating extended aromatic molecules as connectors of DNA strands allows folding of these strands through π-π stacking (DNA 'foldamers'). In previous work it was shown that short oligopyrenotides behave as staircase-like foldamers, which cooperatively self-assemble into 2D supramolecular polymers in aqueous medium. Herein, we demonstrate that 10-mer DNA-sequence conjugated with seven pyrene unites forms dimensionally-defined supramolecular polymers under thermodynamic conditions in water. We present the self-assembly behavior, morphologycal studies (AFM and TEM), and the spectroscopic properties (UV/vis, CD) of the investigated pyrene - conjugated DNA-sequence.

Whole-body motion discrimination in the visually impaired

Visually impaired people show superior abilities in various perception tasks such as auditory attention, auditory temporal resolution, auditory spatial tuning, and odor discrimination. However, with the use of psychophysical methods, auditory and olfactory detection thresholds typically do not differ between visually impaired and sighted participants. Using a motion platform we investigated thresholds of passive whole-body motion discrimination in nine visually impaired participants and nine age-matched sighted controls. Participants were rotated in yaw, tilted in roll, and translated along the y-axis at two different frequencies (0.3 Hz and 2 Hz). An adaptive 3-down 1-up staircase procedure was used along with a two-alternative direction (leftward vs. rightward) discrimination task. Superior performance of visually impaired participants was found in the 0.3 Hz roll tilt condition. No differences between the visually impaired and controls were observed in all other types of motion. The superior performance in the 0.3 Hz roll tilt condition could reflect differences in the integration of extra-vestibular cues and increased sensitivity towards changes in the direction of the gravito-inertial force. In the absence of visual information, roll tilts entail a more pronounced risk of falling, and this could eventually account for the group difference. It is argued that differences in experimental procedures (i.e. detection vs. discrimination of stimuli) explain the discrepant findings across perceptual tasks comparing blind and sighted participants.

Stair climbing - An insight and comparison between women with and without joint hypermobility: A descriptive study.

Generalized joint hypermobility (GJH) is a frequent entity in rheumatology with higher prevalence among women. It is associated with chronic widespread pain, joint dislocations, arthralgia, fibromyalgia and early osteoarthritis. Stair climbing is an important functional task and can induce symptoms in hypermobile persons. The aim of this study was to compare ground reaction forces (GRF) and muscle activity during stair climbing in women with and without GJH. A cross-sectional study of 67 women with normal mobility and 128 hypermobile women was performed. The hypermobile women were further divided into 56 symptomatic and 47 asymptomatic. GRFs were measured by force plates embedded in a six step staircase, as well as surface electromyography (EMG) of six leg muscles. Parameters derived from GRF and EMG were compared between groups using t-test and ANOVA. For GRF no significant differences were found. EMG showed lower activity for the quadriceps during ascent and lower activity for hamstrings and quadriceps during descent in hypermobile women. For symptomatic hypermobile women these differences were even more accentuated. The differences in EMG may point towards an altered movement pattern during stair climbing, aimed at avoiding high muscle activation. However, differences were small, since stair climbing seems to be not demanding.

40Ar-39Ar dating of rocks drilled at sites 60-458 and 60-459

40Ar-39Ar dating of a high-MgO bronzite andesite from near the top of basement drilled at Site 458 shows the characteristic symptoms of artificially disturbed samples - i.e., an inverse staircase-type age spectrum, approximate linearity on an isochron plot, and concordance between total fusion age and isochron age. From conclusions based on other artificially disturbed samples (Ozima et al., 1979), we suggest that the reference isochron age (33.6 Ma) approximates the age of the sample. A basalt from deeper in Hole 458 gives an isochron age of 19.1 ± 0.2 Ma, which is slightly younger than the plateau age of 21.4 ± 1.0 Ma. Both ages are, however, considerably younger than the age of fossils in the overlying sediments (30 - 34 Ma). The age discrepancy may be explained if the 40Ar-39Ar age represents the age of secondary minerals, which formed later. No useful age data were obtained from a basalt sample recovered from Hole 459B.

Bóvedas y contrarresto del Colegio de Nuestra Señora de la Antigua de Monforte de Lemos : geometría, construcción y mecánica

El Colegio de Nuestra Señora de la Antigua de Monforte de Lemos (1593-1619) constituye el principal ejemplo del clasicismo herreriano de Galicia. Esta tesis analiza los aspectos técnicos de la construcción de sus bóvedas de cantería, en el marco disciplinar de la Historia de la Construcción. El Colegio contiene varios tipos de bóvedas entre las que destacan su cúpula trasdosada y su escalera monumental. Además, se conservan los documentos notariales históricos que describen las obras y una de las monteas más importantes de Galicia. Las bóvedas han sufrido movimientos y algunas presentan importantes deformaciones, que han suscitado preocupación sobre su estabilidad. El estudio se organiza en tres epígrafes: geometría, construcción y mecánica, tres aspectos estrechamente interrelacionados en este tipo de estructuras. La parte de estudio geométrico parte de un levantamiento en el que se emplea una estación total sin reflectante, una técnica que permite una medición muy precisa de la forma real de las bóvedas. Como complemento se ha recurrido a técnicas de escáner fotográfico. Con los datos de las mediciones se han elaborado los dibujos de las bóvedas, y analizado sus medidas e irregularidades. Se ha incluido un análisis metrológico por métodos inductivos, que ha puesto de manifiesto la unidad utilizada en la construcción. El estudio se completa con la recopilación de las reglas de dimensionamiento que figuran en los tratados, que se comparan con las dimensiones de las bóvedas analizadas. En el estudio constructivo se registran los despieces y la configuración constructiva y se contrastan, por un lado, con las soluciones que aparecen en los principales tratados de los siglos XVI, XVII y XVIII, y por otro, con las descripciones que se encuentran en los documentos históricos del Colegio. Se ha prestado especial atención al estudio de estos documentos, que han permitido conocer algunos aspectos originales del proceso de construcción de estas bóvedas. En una de las paredes del claustro se ha identificado una montea inédita cuyo levantamiento y análisis se incorpora en esta parte. En el estudio mecánico se analiza la estabilidad dentro del marco teórico del Moderno Análisis Límite de Estructuras de Fábricas. La comprensión del comportamiento estructural, unida a la precisión de los levantamientos, ha permitido explicar los movimientos de las fábricas e inferir la geometría original de algunas de las bóvedas, no siempre evidente en los casos de fuertes deformaciones. Los estudios realizados hasta ahora en bóvedas de los siglos XVI y XVII permiten vislumbrar un panorama de soluciones constructivas más rico que el que recogen los tratados históricos. Las nuevas técnicas de medición sin contacto abren nuevas posibilidades para analizar estas estructuras. Los casos estudiados todavía son escasos y se necesita un conjunto más amplio para poder realizar un análisis general. Esta tesis aporta el estudio de uno de esos casos con el fin de contribuir al mejor conocimiento de la construcción en España en los siglos XVI y XVII. ABSTRACT The Colegio de Nuestra Señora de la Antigua (1593-1619), in Monforte de Lemos, Spain, is the best example in Galicia of the classicism influenced by the Monastery of El Escorial and his architect, Juan de Herrera. This thesis analyzes the technical aspects of the construction of its masonry vaults within the discipline of Construction History. The Colegio exhibits various types of vaults, among which the extradosed ashlar dome and the grand staircase are particularly worth mentioning. In addition, the legal documents containing thorough specifications of the work, as well as one of the construction drawings at actual size —one of the best examples in Galicia—, have both been preserved to this date. Some of the vaults have undergone important deformations that have raised concerns about their stability. The study is organized in three sections: geometry, construction, and mechanics, the three clearly interrelated in this type of structures. The geometrical study starts out with a metric survey using a reflectorless total station, a technique that allows a very precise measurement of the actual shape of the vault. This technique was complemented with the use of a photo-based 3D scanner. The resulting measurements were used to draw the vaults, and analyze their dimensions and irregularities. An inductive metrological analysis, which was able to reveal the exact metric unit utilized during the construction, is included in this section. The section is completed with the gathering of the dimensional rules appearing in the various historical treatises, which are compared with the dimensions of the actual vaults. The construction study deals with the quartering of the structure, whose results are compared, on the one hand, with the approaches appearing in the main treatises of the 16th- 18th centuries, and on the other, with the descriptions in the historical documents of the Colegio itself. Special attention has been paid to the study of these documents, which have revealed some original aspects in the construction process of the vaults. In one of the cloister’s walls, a hitherto unheard-of construction drawing at actual size was found. Its survey and analysis is included in this section. The mechanical study analyzes the Colegio's vaults stability within the theoretical frame of the Modern Limit Analysis of Masonry Structures. The interpretation of the structural behavior, coupled with the precision of the surveys, has allowed a deep understanding of the masonry’s movements and the original geometry of some of the vaults —not always evident in the case of strong deformations— has been inferred. The studies dealing with XVI and XVII century vaults conducted up to this date show a landscape of constructive solutions far richer than the one suggested in the historical treatises. The new contactless measuring techniques offer exciting possibilities for the analysis of these structures. The cases studied are still few and more would be needed before a general analysis can be attempted. This thesis constitutes one such additional case and its goal is to improve our understanding of Spanish construction in the 16th-17th centuries.

La escalera de caracol con ojo a finales del S.XV y comienzos del S.XVI: el caso de la ciudad y tierra de Segovia

Este trabajo de investigación se ha centrado en la escalera de caracol con ojo también denominado caracol de mallorca, en sus primeras apariciones a finales del s.XV y comienzos del s.XVI durante el llamado periodo tardogótico. Este elemento aparentemente sencillo presenta, sin embargo, una gran diversidad de esquemas geométricos. Todavía a día de hoy, no hay explicación del por qué de la mayor difusión de unos con respecto a otros. El objetivo de este trabajo ha sido valorar el papel que el confort/seguridad de uso así como la economía en el trabajo –sencillez de ejecución y coste económico– tuvieron en el contexto de la difusión de este elemento arquitectónico. Este estudio ha analizado en base a estos criterios objetivos, la geometría de cada uno de los esquemas tipo que resuelven la formación del hueco central característico este tipo de escaleras, con la intención de determinar las causas objetivas que pudieron motivar la desigual expansión de los mismos. Una vez determinado el marco teórico, se han establecido dos líneas para abordar el problema. Por un lado se ha planteado el análisis de los modelos teóricos recogidos en los distintos textos de cantería escritos por autores españoles desde finales del s.XVI. Si bien en la época en que se escribieron, la arquitectura gótica como sistema había desaparecido, alguno de sus elementos más característicos, como la escalera de caracol, continuaban empleándose. Las trazas del caracol con ojo recogidas en estos textos, describen habitualmente las soluciones geométricas más frecuentes e incluso los ejemplos construidos más conocidos, los cuales fueron levantados en su mayoría en el período en el cual se desarrolla esta investigación. Por otro lado, este estudio se plantea el análisis de modelos construidos existentes en la Ciudad y Tierra de Segovia. La razón que justifica dichos límites geográficos, la encontramos en que la mayor parte del patrimonio gótico existente en este área fue construido entre finales del s.XV y comienzos del s. XVI. Ambos grupos, modelos teóricos y modelos construidos, recogen la herencia gótica heredada en la construcción de este elemento arquitectónico. Cada una de estas dos líneas establecidas han seguido una metodología específica adaptada al objeto de análisis, si bien ambas se basan en un primer análisis individual y posterior análisis comparativo de los modelos seleccionados. Estos análisis han permitido identificar los distintos esquemas empleados para resolver el hueco central, lo que ha posibilitado establecer una clasificación general de los mismos en tres grupos: caracol con ojo de solución radial, el caracol con ojo de solución no radial y el caracol con ojo de solución tangencial. A partir de los datos y resultados obtenidos del análisis comparativo de los parámetros geométricos que afectan al confort/seguridad de uso y economía en el trabajo de dichos esquemas, podemos concluir que a pesar que los tres esquemas identificados resuelven el caracol con ojo de forma muy similar, el predominio de la solución radial y la caída en el olvido de las soluciones tangencial y no radial estuvieron justificados. ABSTRACT This work focused on the helical staircase, also known as mallorca staircase, in the late 15th and the early 16th centuries. This seemingly simple element, presents however a wide variety of geometrical designs. Still to date, there is no explanation of why some of them were limited use, while others had a wide impact. The aim of this study was to assess the influence which comfort, safety and work economy –ease of construction and financial cost– had on their widespread. This study analysed, following these objective criteria, the geometry of the different designs, which solve the characteristic inner aperture of this type of staircase. And thus, find the reasons, which motivated their uneven spread. Once the theoretical framework was developed, two research lines were set up to address the problem. On one hand, this study analysed the theoretical helical staircases described in the early Spanish texts, which were written since the end of the 16th century. Although these texts were written after the Gothic Architecture system had already disappeared, some of its more representative elements like the helical staircase were still in use. The traces of the helical stair, which are included in these texts, describe the most frequently-used designs and even the most famous real examples which were usually constructed in the Late Gothic period. On the other hand, this study analysed some real helical staircases, which were constructed in the Ciudad y Tierra de Segovia. The reason for choosing these geographical limits is that most of the Gothic buildings of this area were constructed between the end of the 15th and the early 16th centuries, during the Late Gothic period. Both groups, theoretical and constructed samples, collect the Gothic construction tradition of the helical stair. Each of these research lines followed a specific methodology, which was adapted to the research object. Nevertheless, both are based on a first individual analysis of each selected example and a later on comparative one. These analysis allowed the identification of the different layouts which solve the inner aperture of the helical staircase, what made possible to set up a broad classification in three groups, according to the geometrical design strategy that solves the central aperture: the helical staircase by radial solution, the helical staircase by non-radial solution and the helical staircase by tangent solution. The research findings of the comparative analysis of their geometrical parameters, which have an impact in the comfort, safety and work economy, have shown that although all of them solve the helical staircase in a really similar way, the fade into oblivion of the non-radial and tangent approaches was objectively motivated.

Linking topography of its potential surface with the dynamics of folding of a protein model

The “3-color, 46-bead” model of a folding polypeptide is the vehicle for adapting to proteins a mode of analysis used heretofore for atomic clusters, to relate the topography of the potential surface to the dynamics that lead to formation of selected structures. The analysis is based on sequences of stationary points—successive minima, joined by saddles—that rise monotonically in energy from basin bottoms. Like structure-seeking clusters, the potential surface of the model studied here is staircase-like, rather than sawtooth-like, with highly collective motions required for passage from one minimum to the next. The surface has several deep basins whose minima correspond to very similar structures, but which are separated by high energy barriers.

Room-temperature single-electron junction.

The design, realization, and test performances of an electronic junction based on single-electron phenomena that works in the air at room temperature are hereby reported. The element consists of an electrochemically etched sharp tungsten stylus over whose tip a nanometer-size crystal was synthesized. Langmuir-Blodgett films of cadmium arachidate were transferred onto the stylus and exposed to a H2S atmosphere to yield CdS nanocrystals (30-50 angstrom in diameter) imbedded into an organic matrix. The stylus, biased with respect to a flat electrode, was brought to the tunnel distance from the film and a constant gap value was maintained by a piezo-electric actuator driven by a feedback circuit fed by the tunneling current. With this set-up, it is possible to measure the behavior of the current flowing through the quantum dot when a bias voltage is applied. Voltage-current characteristics measured in the system displayed single-electron trends such as a Coulomb blockade and Coulomb staircase and revealed capacitance values as small as 10(-19) F.

Knowledge spirals, situational leadership and informal learning applied on b-learning methodology

The methodology “b-learning” is a new teaching scenario and it requires the creation, adaptation and application of new learning tools searching the assimilation of new collaborative competences. In this context, it is well known the knowledge spirals, the situational leadership and the informal learning. The knowledge spirals is a basic concept of the knowledge procedure and they are based on that the knowledge increases when a cycle of 4 phases is repeated successively.1) The knowledge is created (for instance, to have an idea); 2) The knowledge is decoded into a format to be easily transmitted; 3) The knowledge is modified to be easily comprehensive and it is used; 4) New knowledge is created. This new knowledge improves the previous one (step 1). Each cycle shows a step of a spiral staircase: by going up the staircase, more knowledge is created. On the other hand, the situational leadership is based on that each person has a maturity degree to develop a specific task and this maturity increases with the experience. Therefore, the teacher (leader) has to adapt the teaching style to the student (subordinate) requirements and in this way, the professional and personal development of the student will increase quickly by improving the results and satisfaction. This educational strategy, finally combined with the informal learning, and in particular the zone of proximal development, and using a learning content management system own in our University, gets a successful and well-evaluated learning activity in Master subjects focused on the collaborative activity of preparation and oral exhibition of short and specific topics affine to these subjects. Therefore, the teacher has a relevant and consultant role of the selected topic and his function is to guide and supervise the work, incorporating many times the previous works done in other courses, as a research tutor or more experienced student. Then, in this work, we show the academic results, grade of interactivity developed in these collaborative tasks, statistics and the satisfaction grade shown by our post-graduate students.

Coherent transport in graphene nanoconstrictions

We study the effect of a structural nanoconstriction on the coherent transport properties of otherwise ideal zigzag-edged infinitely long graphene ribbons. The electronic structure is calculated with the standard one-orbital tight-binding model and the linear conductance is obtained using the Landauer formula. We find that, since the zero-bias current is carried in the bulk of the ribbon, this is very robust with respect to a variety of constriction geometries and edge defects. In contrast, the curve of zero-bias conductance versus gate voltage departs from the (2n+1)e2∕h staircase of the ideal case as soon as a single atom is removed from the sample. We also find that wedge-shaped constrictions can present nonconducting states fully localized in the constriction close to the Fermi energy. The interest of these localized states in regards to the formation of quantum dots in graphene is discussed.