would produce a non-adiabatic torque of the form given by the second term in Eq. (\[lt\]). The correctness of this predicted non-adiabatic torque depends on the correctness of the assumed model for relaxation of transverse spin accumulation through spin flip scattering.

Zhang and Li assume a form for the rate of spin flip scattering, $\delta {\bf m}
/\tau_{\rm sf}$, that has been used successfully as a phenomenological description of [*longitudinal*]{} spin relaxation in systems with collinear magnetization. While it is plausible to extend this form, as they do, to describe [*transverse*]{} spin relaxation in non-collinear systems, our calculations indicate that it is not likely to be correct. Our reasoning is simplest to appreciate for a spin spiral with small pitch $p$. In this limit, Eqs. (\[alpha\]) and (\[eqn:spirald\]) show that the transverse component of the spin for every electron eigenstate is proportional to its velocity. This means that the majority band electrons contribute a transverse spin accumulation and an electric current that are proportional to one other. The same is true, separately, for the minority band electrons. This conclusion is independent of the details of the electron distribution. Therefore, for a fixed total current, it is impossible to relax the 