  We have studied the zero magnetic field resistivity of unique high- mobility
two-dimensional electron system in silicon. At very low electron density (but
higher than some sample-dependent critical value, $n_{cr}\sim 10^{11}$
cm$^{-2}$), CONVENTIONAL WEAK LOCALIZATION IS OVERPOWERED BY A SHARP DROP OF
RESISTIVITY 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 $n_s<N_{cr}$, the sample is insulating. The resistivity is empirically
found to SCALE WITH TEMPERATURE BOTH BELOW AND ABOVE $n_{cr}$ WITH A SINGLE
PARAMETER which approaches zero at $n_s=n_{cr}$ suggesting a metal/ insulator
phase transition.
