Suppression of metallic behavior in two dimensions by spin flip scattering

We study the effect of the disorder on the metallic behavior of a two-dimensional electron system in silicon. The temperature dependence of conductivity $\sigma (T)$ was measured for different values of substrate bias, which changes both potential scattering and the concentration of disorder-induced local magnetic moments. We find that the latter has a much more profound effect on $d\sigma/dT$. In fact, the data suggest that in the limit of $T\to 0$ the metallic behavior, as characterized by $d\sigma/dT < 0$, is suppressed by an arbitrarily small amount of spin flip scattering by local magnetic moments.