We report weak localization studies of quantum coherence in metal nanowires with widths as small as 5 nm, demonstrating that structures fabricated at sub-50 nm length scales can reveal coherence phenomena not accessible in larger devices. Through selective etching of cleaved molecular-beam epitaxy (MBE)-grown substrates, we produce precise nanoscale surface relief then used as a stencil for metal deposition. This nonlithographic method of lateral definition allows the fabrication of metal (AuPd) nanowires greater than one micron in length with widths below 5 nm, a previously unexplored size regime in studies of quantum corrections to the conductance of disordered metals. Analyzing magnetoresistance data, we find that the coherence time, T
φ, shows a low temperature T dependence close to quasi-1D theoretical expectations (T
φ ∼ T-2/3 in 5 nm wide wires, while exhibiting a relative saturation as T 0 for wide samples of the same material. Since an externally controlled parameter, the sample geometry, can cause a single material to exhibit both suppression and divergence ofT
φ, this finding provides a new constraint on models of dephasing phenomena.