Bias and temperature dependence of the 0.7 conductance anomaly in quantum point contacts

The $0.7{(2e}^2/h)$ conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at $0.85{(2e}^2/h)$ at finite bias. On varying the gate voltage the transition between the 1.0 and $0.85{(2e}^2/h)$ plateaus occurs for definite bias voltages, which define a gate-voltage-dependent energy difference $\Delta .$ This energy difference is compared with the activation temperature $T_a$ extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find $\Delta {=k}_B{T}_a,$ which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge $\Delta$ below the chemical potential becomes thermally depopulated as the temperature is increased.