This semiclassical calculation was performed in order to relax the assumption of adiabatic transport in the point contact region, and of small Td/N, on which Eqs. (3.32) and (3.34) are based. The dashed curve is for point contacts defined by hardwall contours with straight corners (no collimation); the dots are for the smooth hardwall contours shown in the inset, which lead to collimation via the horn effect