  In the ballistic regime, the metallic temperature dependence of the
conductivity in a two-dimensional electron system in silicon is found to change
non-monotonically with the degree of spin polarization. In particular, it fades
away just before the onset of complete spin polarization but reappears again in
the fully spin-polarized state, being, however, suppressed relative to the
zero-field case. Analysis of the degree of the suppression allows one to
distinguish between the screening and the interaction-based theories.
