  The temperature dependence of conductivity $\sigma (T)$ in the metallic phase
of a two-dimensional electron system in silicon has been studied for different
concentrations of local magnetic moments. The local moments have been induced
by disorder, and their number was varied using substrate bias. 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 scattering by
local magnetic moments.
