Abstract
For five different electron and hole systems in two dimensions (Si MOSFETs, p-GaAs, p-SiGe, n-GaAs and n-AlAs), the critical density nc that marks the onset of strong localization is shown to be a single power law function of the scattering rate 1/τ 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 nc → 0 in the limit of infinite mobility.