Quantum electronic transport through three-dimensional microconstrictions with variable shapes.

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 ~1990!#. 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 section!. In a weak longitudinal magnetic field, when rc@d , where rc 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, rc!d , 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.