  III-V nanowires are comprehensively studied because of their suitability for
optoelectronic quantum technology applications. However, their small dimensions
and the spatial separation of carriers from the wire surface render electrical
contacting difficult. Systematically studying ohmic contact formation by
diffusion to $n$-doped GaAs nanowires, we provide a set of optimal annealing
parameters for Pd/Ge/Au ohmic contacts. We reproducibly achieve low specific
contact resistances of $\sim2\times10^{-7}\,\Omega\text{cm}^2$ at room
temperature becoming an order of magnitude higher at $T\simeq4.2\,$K. We
provide a phenomenological model to describe contact resistances as a function
of diffusion parameters. Implementing a transfer-matrix method, we numerically
study the influence of the Schottky barrier on the contact resistance. Our
results indicate that contact resistances can be predicted using various
barrier shapes but further insights into structural properties would require a
full microscopic understanding of the complex diffusion processes.
