  A dc current induced in a quantum constriction by a traveling acoustic wave
(or by non-equilibrium ballistic phonons) is considered. We show that in many
important situations the effect is originated from acoustically-induced
scattering between the propagating and reflecting states in the constriction.
Two particular regimes corresponding to relatively high and low acoustic
frequencies are discussed. In the first regime, the acoustoelectric effect in a
smooth constriction can be understood by semi-classical considerations based on
local conservation laws. For the low frequency regime, we show that the
acousto-conductance is closely related to the zero field conductance. The
qualitative considerations are confirmed by numerical calculations both for
smooth and abrupt channels.
