  We present a theory of voltage-induced spin-transfer torques in ballistic
non-collinear spin valves. The torkance on one ferromagnetic layer is expressed
in terms of scattering coefficients of the whole spin valve, in analogy to the
Landauer conductance formula. The theory is applied to Co/Cu/Ni(001)-based
systems where long-range oscillations of the Ni-torkance as a function of Ni
thickness are predicted. The oscillations represent a novel quantum size effect
due to the non-collinear magnetic structure. The oscillatory behavior of the
torkance contrasts a thickness-independent trend of the conductance.
