A protocol for the characterization of thin conducting films by means of terahertz time-domain spectroscopy (THz-TDS) is thoroughly presented. By employing a Gires–Tournois étalon configuration in reflection mode, it is shown that amplitude-only measurements are sufficient to accurately characterize the sheet resistance of thin films at THz frequencies thanks to the high sensitivity of the proposed technique. A large series of samples of nanometric thickness that span a broad range of sheet resistance values are characterized: fluorine-doped zinc oxide, aluminum-doped zinc oxide, nanometric titanium and reduced graphene oxide films, tungsten and platinum diselenide. In addition, a flexible THz perfect absorber based on a Salisbury screen is experimentally demonstrated in the context of the analysis. The results confirm that THz-TDS in reflection mode provides a powerful tool for the fast and nondestructive …