Metal nanowires (NWs) with a diameter of a few nanometers have attracted considerable attention as a promising one-dimensional nanomaterial due to their inherent flexibility and conductive properties and their weak plasmon absorption in the visible region. In a previous paper, we reported the synthesis of ultrathin 1.8 nm-diameter Au NWs using toluene-solubilized aqueous solutions of a long-chain amidoamine derivative (C18AA). This study investigates the effect of different organic solvents solubilized in C18AA aqueous solutions on the morphology of the Au products and demonstrates that solubilizing methylcyclohexane yields thick 2.7 nm-diameter Au NWs and 3.3 nm-diameter Au–Ag alloy NWs. Further, the surface-enhanced Raman scattering sensitivity of ultrathin Au NWs, thick Au NWs, and thick Au–Ag alloy NWs were assessed using 4-mercaptopyridine and found that their enhancement factors are 10⁴–10⁵ and the order is Au–Ag NWs > thick Au NWs > ultrathin Au NWs.