The transmission behavior of a quantum point contact in a magnetic field is calculated, exhibiting in detail the influence of intersubband scattering. In contrast to the commonly known reduction of backscattering by a magnetic field, it is shown here that a weak field can cause increased backscattering. Nevertheless, the present results serve to illustrate that backscattering can occur without any effect on the conductance quantization, which is explained by an indirect compensation mechansim.