We have studied the current-voltage characteristics of superconducting weaklinks in which the coupling medium is the 2-D electron gas in InAs-basedsemiconductor quantum wells, with relatively large (typically 0.5 micron)separations between niobium electrodes. The devices exhibit Josephson-likecurrent-voltage characteristics; however, the falloff of the differentialresistance with decreasing temperature is thermally activated, and is orders ofmagnitude slower than for more conventional weak links. Most unexpectedly, thethermal activation energies are found to be proportional to the width of thedevice, taken perpendicular to the current flow. This behavior falls outsidethe range of established theories; we propose that it is a fluctuation effectcaused by giant shot noise associated with multiple Andreev reflections. Thepossibility of non-equilibrium effects is discussed.
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