  Using a combination of ground state quantum Monte-Carlo and finite size
scaling techniques, we perform a systematic study of the effect of Coulomb
interaction on the localisation length of a disordered two-dimensional electron
gas. We find that correlations delocalise the 2D system. In the absence of
valley degeneracy (as in GaAs heterostructures), this delocalization effect
corresponds to a finite increase of the localization length. The delocalisation
is much more dramatic in the presence of valley degeneracy (as in Si MOSFETSs)
where the localization length increases drastically. Our results suggest that a
rather simple mechanism can account for the main features of the metallic
behaviour observed in high mobility Si MOSFETs. Our findings support the claim
that this behaviour is indeed a genuine effect of the presence of
electron-electron interactions, yet that the system is not a ``true'' metal in
the thermodynamic sense.
