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Superconductors coupled with a two-dimensional electron gas in GaAs/AlGaAs and InAs/AlGaSb heterostructures (Received 10 May lYY3; accepted for publication I7 June IYYj) We fabricated superconducting Sn/Pd contacts to a two-dimensional electron gas (ZDEG) in GaAs/AlGaAa heterostructurcs by an alloying technique. A dip around zero bias and peaks at finite voltages have been observed in the differential resistance of a 2DEG between two superconductors. The resistance dip may be a result of Andrccb reflection at 2DEG-superconductor intcrfaccs and the peaks arc probably due to the Josephson coupling between two very close superconducting islands in a superconducting contact region. which may bc formed by alloying. Alternatively, we made superconducting Nb contacts to a 2DEG in an InAs/AlGaSb quantum well structure. The resistance of Nb-2DEG-Nb devices shows a clear indication of Andrccv reflection. 1