  We demonstrate a scanning gate grid measurement technique consisting in
measuring the conductance of a quantum point contact (QPC) as a function of
gate voltage at each tip position. Unlike conventional scanning gate
experiments, it allows investigating QPC conductance plateaus affected by the
tip at these positions. We compensate the capacitive coupling of the tip to the
QPC and discover that interference fringes coexist with distorted QPC plateaus.
We spatially resolve the mode structure for each plateau.
