Solid-state electrochemical switches, also known as Quantised Conductance Atomic Switches (QCAS), have been investigated using reversible conversion of silver to silver sulphide for the switching. Electron-beam lithography and a custom plasma-based sulphidation technique have been employed for device fabrication, and optical ellipsometry has been used to accurately characterise the sulphidation reaction. The sulphide layer that is formed saturates at a thickness of 10-12 nm, which is ideal for these devices. Prototype devices, fabricated using nichrome as the inert electrode, have an OFF/ON resistance ratio of 1000, with an on-state resistance of 90 Ω.