  For five different electron and hole systems in two dimensions (Si MOSFET's,
p-GaAs, p-SiGe, n-GaAs and n-AlAs), the critical density, $n_c$ that marks the
onset of strong localization is shown to be a single power-law function of the
scattering rate $1/\tau$ deduced from the maximum mobility. The resulting curve
defines the boundary separating a localized phase from a phase that exhibits
metallic behavior. The critical density $n_c \to 0$ in the limit of infinite
mobility.
