Weak localization in a GaAs heterostructure close to population of the second subband

Weak-localization magnetoresistance has been measured as a function of carrier density using a back-gate technique. The phase relaxation rate and the spin-orbit relaxation rate have been determined. The dominating contribution to the phase relaxation rate comes from electron-electron interaction, as demonstrated for samples covering densities from 2.5 to 7.5×{}10¹⁵ m^-2. Interband scattering is found to enhance phase relaxation. A strictly two-dimensional electron gas (2DEG) quenches the spin-orbit relaxation rate in the plane of the 2DEG, 1/τₛₒx,y, whereas the orthogonal part, 1/τₛₒᶻ, appears added to the phase relaxation rate in the interpretation of the weak-localization magnetoresistance, and can be determined as the saturation value of the phase relaxation at low temperatures. For high carrier densities, intersubband scattering makes the spin-orbit scattering more isotropic and our data support a theory by Elliott.