We have performed measurements on a semiconductor quantum wire in which we induce a local potential perturbation with the metallic tip of a scanning force microscope. Measurement of the sample resistance as a function of tip position results in an electrical map of the wire in real space. We find the fingerprint of potential fluctuations in the wire which appear as local resistance fluctuations in the images. In a local transconductance measurement we observe small oscillations on the scale of the Fermi-wavelength of electrons which may arise from interference of electron waves.