Silicon nanowires assembled from clusters or etched from the bulk, connected to aluminum electrodes and passivated, are studied with large-scale local-density-functional simulations. Short ( approximately 0.6 nm) wires are fully metallized by metal-induced gap states resulting in finite conductance ( approximately e(2)/h). For longer wires ( approximately 2.5 nm) nanoscale Schottky barriers develop with heights larger than the corresponding bulk value by 40% to 90%. Electric transport requires doping dependent gate voltages with the conductance spectra exhibiting interference resonances due to scattering of ballistic channels by the contacts.

The transport properties of three-dimensional quantum microconstrictions in field-free conditions and under the influence of magnetic fields of arbitrary strengths and directions are studied via a generalized Büttiker model ͓Phys. Rev. B 41, 7906 ͑1990͔͒. It is shown that conductance quantization is influenced by the geometry of the microconstriction ͑that is, its length and the shape of its transverse cross section͒. In a weak longitudinal magnetic field, when r c ӷd, where r c is the cyclotron radius and d the effective transverse size of the narrowing of the microconstriction, the conductance exhibits Aharonov-Bohm-type behavior. This behavior transforms in the strong-field limit, r c Ӷd, into Shubnikov-de Haas oscillations with a superimposed Aharonov-Bohm fine structure. The dependence of the Aharonov-Bohm-type features on the length of the microconstriction and on temperature are demonstrated. Transverse magnetic fields lead to depopulation of the magnetoelectric subbands, resulting in a steplike decrease of the conductance upon increasing the strength of the applied magnetic field.

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Noise in the thermal current through a ballistic quantum wire is considered both for noninteracting and interacting particles. It is shown that for a perfect quantum wire the equilibrium thermal ͑Johnson-Nyquist͒ noise does not depend on the statistics of the heat carriers. In contrast, the nonequilibrium noise, produced by the temperature difference between the heat baths, is different for fermions and bosons. The general expressions which are obtained, are used in calculations of the noise power of the thermal current through a Luttinger liquid wire connected to reservoirs of noninteracting electrons.

A theoretical analysis of thermal transport in nanowires, in field-free conditions and under influence of applied magnetic fields, is presented. It is shown that in the nonlinear regime ͑finite applied voltage͒ new peaks in the Peltier coefficient appear leading to violation of Onsager's relation between the Peltier and thermopower coefficients. Oscillations of the Peltier coefficient in a magnetic field are demonstrated. The thermoconductance has a steplike quantized structure similar to the electroconductance and it exhibits deviations from the Wiedemann-Franz law. The strong dependence of the thermoconductance on the applied magnetic field leads to the possibility of magnetic blockade of thermal transport in wires with a small number of conducting channels. Possible control of thermal transport in nanowires through external parameters, that is applied through finite voltages and magnetic fields, is discussed. ͓S0163-1829͑99͒04140-5͔