Instability of the two-dimensional metallic phase to a parallel magnetic field

Magnetotransport studies of the unusual two-dimensional metallic phase in high-mobility Si-MOS structures are reported. It is found that a magnetic field applied in the 2D plane suppresses the metallic state, causing the resistivity to increase dramatically (by more than 30 times). The total existence range of the metallic state is found to contain three distinct types of magnetoresistance, related to the corresponding quantum corrections to the conductivity. The data suggest that the unusual metallic state is a consequence of both spin and Coulomb interaction effects.