Using DMSO as a solvent improves the writing resolution as compared to aqueous electrolytes, allowing much higher confinement for experimentally accessible pulse lengths, due to a lower diffusion coefficient of the ions in solution. Our results show that reproducible writing of metal structures of ~50 nm linewidth is feasible with nanosecond voltage pulses. The diffuse layer charging description can provide a handle for a priori quantitative predictions on the writing resolution and the spatial extent of a potential pulse in nanoscale electrochemical systems. By demonstrating the controlled electrodeposition of sub-100 nm lines, such scanning probe deposition methods are ultimately envisioned to enable the direct maskless writing of 2D and 3D metal or semiconductor nanostructures.