Can the conductance of an adiabatic ballistic constriction be lower than $2e^2/h$?

TL;DR

This study demonstrates that the conductance of a ballistic 1D constriction can be lower than the expected quantized value of 2e^2/h, due to subband shifts caused by gate voltage, supported by experimental and modeling results.

Contribution

First experimental observation of conductance below 2e^2/h in an adiabatic ballistic constriction, explained by subband shifts with gate voltage.

Findings

Conductance decreases from 2e^2/h to 0.97×2e^2/h with negative gate voltage.

The decrease is consistent with analytical and realistic device modeling.

Subband shifts cause the conductance reduction in the 1D constriction.

Abstract

We have performed four-terminal conductance measurements of a one-dimensional (1D) channel in which it is possible to modulate the potential profile using three overlaying finger gates. In such a 1D ballistic structure we have observed, {\em for the first time,} that the conductance steps show a gradual decrease from $2e^2/h$ to $0.97 \times 2e^2/h$ with increasing negative finger gate voltage in a short, clean 1D constriction. We suggest this phenomenon is due to differing shifts of 1D subbands with changing spilt-gate voltage. Both a simple analytical estimate for an adiabatic constriction and, realistic modeling of the device, give the same magnitude of the conductance decrease as observed in our experiments.