We present the first study of a new system where the confinement potential of a high mobility quasi‐one‐dimensional electron gas on the (100) GaAs surface is varied using two, two‐dimensional hole gases produced on the adjacent (311)A surfaces. The structure consists of two lateral two‐dimensionalp‐njunctions, placed back‐to‐back to form ap‐n‐pstructure. The confinement potential of the narrown‐type channel can thus be modulated by applying a bias to the adjacentp‐type regions. Magnetoresistance measurements of the narrow channel show magnetic depopulation of the one‐dimensional subbands [K.‐F. Berggren, T. J. Thornton, D. J. Newson, and M. Pepper, Phys. Rev. Lett.57, 1769 (1986)], following the model of Berggrenetal. [Phys. Rev. B37, 10118 (1988)] widths and one‐dimensional carrier concentrations are extracted to fully characterize the dependence of the channel on the applied ‘‘hole‐gate’’ voltage. ©1996 American Institute of Physics.