  We have varied the disorder in a two-dimensional electron system in silicon
by applying substrate bias. When the disorder becomes sufficiently low, we
observe the emergence of the metallic phase, and find evidence for a
metal-insulator transition (MIT): the single-parameter scaling of conductivity
with temperature near a critical electron density. We obtain the scaling
function $\beta$, which determines the length (or temperature) dependence of
the conductance. $\beta$ is smooth and monotonic, and linear in the logarithm
of the conductance near the MIT, in agreement with the scaling theory for
interacting systems.
