Classical versus Quantum Effects in the B=0 Conducting Phase in Two   Dimensions
  In the dilute two-dimensional electron system in silicon, we show that the temperature below which Shubnikov-de Haas oscillations become apparent is approximately the same as the temperature below which an exponential decrease in resistance is seen in B=0, suggesting that the anomalous behavior in zero 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. 
S. V. Kravchenko, D. Simonian, K. Mertes, M. P. Sarachik, and T. M.   Klapwijk