On the metallic behaviour in dilute two-dimensional systems
  We have studied the metallic behaviour in low density GaAs hole systems, and Si electron systems, close to the apparent two-dimensional metal-insulator transition. Two observations suggest a semi-classical origin for the metallic-like behaviour. Firstly the strength of the metallic behaviour is almost independent of the symmetry of the confining potential, and is predominantly determined by the low temperature resistivity (i.e. by k_F*l). Secondly the fractional decrease in conductivity with increasing temperature depends only on T/T_F, independent of the carrier density, in both systems. 
A.R. Hamilton, M.Y. Simmons, M. Pepper and D.A. Ritchie