Theory of the conductance of parallel ballistic constrictions

We present exact quantum-mechanical calculations of the conductance G of parallel ballistic constrictions (BC) connecting two semi-infinite two-dimensional electron gases. The dependence of G on the length and width of each BC, on the separation between them, on their shape, and on the Fermi energy is described. We show that the conductance of the BC's is nearly additive under most conditions because of the continuous distribution of incident modes and electron-beam collimation along the constrictions. Deviations from additivity due to coherence effects occur only for very short and wide constrictions.