  Ballistic electrons flowing through a constriction can transfer momentum to
the lattice and excite a vibration of a free-standing conductor. We show (both
numerically and analytically) that the electromechanical noise power P does not
vanish on the plateaus of quantized conductance -- in contrast to the current
noise. The dependence of $P$ on the constriction width can be oscillatory or
stepwise, depending on the geometry. The stepwise increase amounts to an
approximate quantization of momentum noise.
