QUANTUM ELECTRONIC TRANSPORT THROUGH 3-DIMENSIONAL MICROCONSTRICTIONSWITH VARIABLE SHAPES

The transport properties of three-dimensional quantum microconstrictio ns in held-free conditions and under the influence of magnetic fields of arbitrary strengths and directions are studied via a generalized Bu ttiker model [Phys. Rev. B 41, 7906 (1990)]. It is shown that conducta nce quantization is influenced by the geometry of the microconstrictio n (that is, its length and the shape of its transverse cross section). In a weak longitudinal magnetic field, when r(c) much greater than d, where r(c) is the cyclotron radius and d the effective transverse siz e of the narrowing of the microconstriction, the conductance exhibits Aharonov-Bohm-type behavior. This behavior transforms in the strong-fi eld limit, r(c) much less than d, into Shubnikov-de Haas oscillations with a superimposed Aharonov-Bohm fine structure. The dependence of th e Aharonov-Bohm-type features on the length of the microconstriction a nd 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 appli ed magnetic field.