It is instructive to first consider

It is instructive to first consider classical 2D point contacts in some detail.31,311 The ballistic electron flow through a point contact is illustrated in Fig. 45a in real space, and in Fig. 45b in k-space, for a small excess electron density δn at one side of the point contact. At low temperatures this excess charge moves with the Fermi velocity vF. The flux normally incident on the point contact is δnvFhcos φ θ(cos φ)i, where θ(x) is the unit step function and the symbol h i denotes an isotropic angular average (the angle φ is defined in Fig. 45a). In the ballistic limit l 佲 W the incident flux is fully transmitted, so the total diffusion current J through the point contact is given by