The fabrication and characterization of metallic nanometer-sized gaps suitable for conductivity measurements of single molecules were studied. Controlled gap formation by electromigration (EM) is demonstrated in contiguous and ultrathin Ag structures wetting the Si(100) substrate. The gaps obtained are in the range of nanometers or even subnanometers, as revealed by lateral conductivity measurements and scanning tunneling microscopy carried out under ultrahigh vacuum conditions. Annealing to 300 K closes the gap by enabling surface diffusion of Ag, and another cycle of opening by EM at 80 K can be performed. The functionality of the contacts is demonstrated by insertion of ferrocenedithiol molecules into the gap. © 2006 American Institute of Physics.