  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.
