THEORY OF CONDUCTION THROUGH NARROW CONSTRICTIONS IN A 3-DIMENSIONAL ELECTRON-GAS

An exact calculation of the quantum conduction through a curvilinear c onstriction in a three-dimensional electron ga is presented. We show t hat the conductance behavior presents significant differences with res pect to the two-dimensional case. Importantly, we find that the conduc tance of a circular point contact deviates from the classical Sharvin result and the conductance per unit area is not constant except in the limit of macroscopic areas. We show that quantum finite-size effects can be taken into account by a simple semiclassical correction to the Sharvin formula. Recent experiments and calculations on quantum constr ictions formed in atomic-scale point contacts are discussed.