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 $0.5 G_0$, $0.8 G_0$ or $0.9 G_0$
($G_0 = 2e^2/h$), 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 $G_0$. For the QPC tuned to have the anomaly at
$0.5 G_0$ the channel asymmetry is found to be as large as 67% with the spin
gap energy gradually evolving as the conductance increases.