Effects of a single scatterer on the transport properties of a leaky electron waveguide are investigated theoretically. The delta -function is used to simulate the scattering potential. The results show that the presence of even a single scatterer located in the waveguide will lead to obvious distortion of the shape of conductance steps, and will greatly influence the oscillations of the tunnelling current observed in clean waveguides. However, the effects of the scatterer being located outside the tunnelling barrier on either the conductance steps or the oscillations of the tunnelling current are negligible.