Summary form only given. It is shown that a severe degradation in the quantum-effect features occurs when only a single impurity exists in the device. Transport in different lateral regions of the waveguide is studied for a 0.5- mu m-long, 0.3- mu m-wide waveguide. As the waveguide is opened to include a single impurity, an immediate and drastic degradation in the conductance steps is observed, because the presence of the impurity in the channel makes the transmission probability energy dependent and less than unity. By analyzing which 2e/sup 2//h conductance step becomes distorted, the location of two single impurities is pinpointed. One lies very near the thin middle common barrier, while the other lies closer to the outer side barrier. Moreover, in a waveguide containing two impurities, the familiar oscillations in the tunneling current corresponding to the sweeping of the 1D subbands through the Fermi level are completely washed out, due to mode mixing in the vicinity of the impurities.
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