Classical versus quantum effects in the B=0 conducting phase in two dimensions

In a dilute two-dimensional electron system in silicon, we show that the te mperature below which Shubnikov-de Haas oscillations become apparent is app roximately the same as the temperature below which an exponential decrease in resistance is seen in B=0, suggesting that the anomalous behavior in zer o field is observed only when the system is in a degenerate (quantum) state . The temperature dependence of the resistance is found to be qualitatively similar in B=0 and at integer Landau-level filling factors. [SO163-1829(99 )50220-8].