  The temperature dependence of conductivity $\sigma (T)$ of a two-dimensional
electron system in silicon has been studied in parallel magnetic fields B. At
B=0, the system displays a metal-insulator transition at a critical electron
density $n_c(0)$, and $d\sigma/dT >0$ in the metallic phase. At low fields
($B\lesssim 2$ T), $n_c$ increases as $n_c(B) - n_c(0) \propto B^{\beta}$
($\beta\sim 1$), and the zero-temperature conductivity scales as $\sigma
(n_s,B,T=0)/\sigma (n_s,0,0)=f(B^{\beta}/\delta_n)$ (where
$\delta_n=(n_s-n_c(0))/n_c(0)$, and $n_s$ is electron density) as expected for
a quantum phase transition. The metallic phase persists in fields of up to 18
T, consistent with the saturation of $n_c$ at high fields.
