We present for the first time TEM cross-section studies of Au nanowires fabricated with nanostencil membranes. Au metallic nanowires from 100 to 20 nm in width are fabricated on metallic, semiconducting and insulating substrates. The observed profiles require consideration of wetting for a satisfactory understanding. This length scale is easily accessible by simulations, thereby making it possible to model the cross-section profiles. In addition, the fabrication capabilities of the nanostencil technique are thus critically evaluated for the fabrication of metallic nanowires on different substrates up to 20 nm linewidths. We find a clear separation between spreading-dominated and wetting-dominated behaviour. Substrates such as HOPG and mica aid spreading. The 1:1 geometrical reproduction rule is well respected in the wetting case down to the 20 nm limit, before the clogging of the mask on all other substrates.