  We report a precise experimental study on the shot noise of a quantum point
contact (QPC) fabricated in a GaAs/AlGaAs based high-mobility two-dimensional
electron gas (2DEG). The combination of unprecedented cleanliness and very high
measurement accuracy has enabled us to discuss the Fano factor to characterize
the shot noise with a precision of 1 %. We observed that the shot noise at zero
magnetic field exhibits a slight enhancement exceeding the single particle
theoretical prediction, and that it gradually decreases as a perpendicular
magnetic field is applied. We also confirmed that this additional noise
completely vanishes in the quantum Hall regime. These phenomena can be
explained by the electron heating effect near the QPC, which is suppressed with
increasing magnetic field.
