2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/147483We explore the possibility of using a double-tip STM to probe the single electron Green function of a sample surface, and describe a few important applications: (1) Probing constant energy surfaces in $\k$-space by ballistic transport; (2) Measuring scattering phase shifts of defects; (3) Observing the transition from ballistic to diffusive transport to localization; and (4) Measuring inelastic mean free paths.Minor changes, to appear in Phys.Rev.B, 12 pages + 2 figuresCondensed Mattertext