  We present low temperature magnetoresistance measurements of Ar-irradiated
SrTiO3 under an applied electrostatic field. The electric field, applied
through a back gate bias, modulates both the mobility and sheet density, with a
greater effect on the former. For high mobilities, 3-dimensional orbital
magnetoresistance is observed. For low mobilities, negative magnetoresistance
that is consistent with the suppression of 2-dimensional weak-localization is
observed. The crossover from 3 to 2-dimensional transport arises from a
modulation in the carrier confinement, which is enhanced by the electric field
dependent dielectric constant of SrTiO3. The implications of our results on the
development of oxide electronic devices will be discussed.
