Possible metal-insulator transition atB=0 in two dimensions

We have studied the zero magnetic field resistivity {ρ} of unique high-mobility two-dimensional electron systems in silicon. At very low electron density nₛ (but higher than some sample-dependent critical value ncr{~}10¹¹ cm^-2), conventional weak localization is overpowered by a sharp drop of {ρ} by an order of magnitude with decreasing temperature below {~}1--2 K. No further evidence for electron localization is seen down to at least 20 mK. For nₛ{n}cr$, the sample is insulating. The resistance is empirically found to scale with temperature both below and above ${n}cr$ with a single parameter that approaches zero at ${n}ₛ$=${n}cr$ suggesting a metal-insulator phase transition.