  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.
