  We propose a mechanism for the recently reported destabilization by an
in-plane magnetic field of the conducting phase of low density electrons in 2D.
We apply our self-consistent approach based on the memory function formalism to
the fully spin polarized electron system. This takes into account both disorder
and exchange-correlation effects. We show that spin polarization significantly
favors localization because of the enhancement of the exchange-correlations. A
key outcome is that the conducting phase for the fully spin polarized system is
significantly suppressed. The in-plane magnetic field needed to stabilize the
fully spin polarized state lies in the range 0.1<H<1 T, depending on the
carrier density. We determine the metal-insulator phase diagram for the
unpolarized and fully polarized systems, and we estimate the dependence of the
critical magnetic field on carrier density.
