POSSIBLE METAL-INSULATOR-TRANSITION AT B=0 IN 2 DIMENSIONS

We have studied the zero magnetic field resistivity rho of unique high -mobility two-dimensional electron systems in silicon. At very low ele ctron density n(s) (but higher than some sample-dependent critical val ue n(cr) similar to 10(11) cm(-2)), conventional weak localization is overpowered by a sharp drop of rho by an order of magnitude with decre asing temperature below similar to 1-2 K. No further evidence for elec tron localization is seen down to at least 20 mK. For n(s) < n(cr), th e sample is insulating. The resistance is empirically found to scale w ith temperature both below and above n(cr) with a single parameter tha t approaches zero at n(s) = n(cr) suggesting a metal-insulator phase t ransition.