Ballistic electron emission microscopy is used to investigate the electron scattering properties of undamaged and focused ion beam implanted Au/GaAs diodes. Implanted regions show decreased ballistic electron transmission attributed to increased scattering. A quantitative model of electron transport is developed that includes quantum mechanical transmission at the interface, scattering effects from optical phonons and implantation induced defects in the semiconductor, and scattering in the metal layer. Model predictions of scattering in undamaged regions show good agreement with the data. However, we conclude additional scattering effects must be included to describe the implanted structures.