  The conductance quantization and shot noise below the first conductance
plateau $G_0 = 2e^2/h$ are measured in a quantum point contact fabricated in a
GaAs/AlGaAs tunnel-coupled double quantum well. From the conductance
measurement, we observe a clear quantized conductance plateau at $0.5G_0$ and a
small minimum in the transconductance at $0.7 G_0$. Spectroscopic
transconductance measurement reveals three maxima inside the first diamond,
thus suggesting three minima in the dispersion relation for electric subbands.
Shot noise measurement shows that the Fano factor behavior is consistent with
this observation. We propose a model that relates these features to a
wavenumber directional split subband due to a strong Rashba spin--orbit
interaction that is induced by the center barrier potential gradient of the
double-layer sample.
