We report the experimental measurements of dynamic quantum transport in a quantum point contact structure. Both the real and the imaginary parts of the admittance show significant steps, and the imaginary parts of the admittance reflect the inductive response of the time-dependent oscillating voltage. However, the plateaus around zero magnetic field are not perfect flat due to backscattering. The external magnetic field can effectively suppress backscattering and improve the quantity of the quantized steps. The flatter plateaus are observed when the magnetic field extends to 0.3 T. Continuing to increase the magnetic field, the magnetic depletion effect will play role, and thus the sub-band spacing of the zero magnetic field can be estimated.
