Conductance anomaly and Fano factor reduction in quantum point contacts

We report an experimental study on the shot noise as well as the dc transport properties of a quantum point contact (QPC) whose conductance anomaly can be tuned electrostatically by the gate electrodes. By controlling the single QPC so that it has no anomaly or an anomaly at 0.5, 0.8 or 0.9G₀ (G₀=2e²/h), we prove that the anomaly always accompanies the Fano factor reduction due to the asymmetric transmission of the two spin-dependent channels for the conductance lower than G₀. For the QPC tuned to have the anomaly at 0.5G₀ the channel asymmetry is found to be as large as 67% with the spin gap energy gradually evolving as the conductance increases.