Exact quantitative understanding of plasmon propagation along nanowires is mandatory for designing and creating functional devices. Here we investigate plasmon transmission through top-down fabricated monocrystalline gold nanowires on a glass substrate. We show that the transmission through finite-length nanowires can be described by Fabry-Pérot oscillations that beat with free-space propagating light launched at the incoupling end. Using an extended Fabry-Pérot model, experimental and simulated length dependent transmission signals agree quantitatively with a fully analytical model. ; Comment: 5 pages, 4 figures