The electrons of the surface states on the (I 11) surfaces of the noble metals An, Ag, and Cu form a quasi-two-dimensional (2D) free electron gas which is confined to the first few atomic layers at the crystal surface. They are scattered by the potential associated with surface defects, e.g. impurity atoms, adatoms, or step edges, leading to quantum-interference patterns in the local density of states around these defects, We have used the quantum-interference phenomena to quantitatively measure the electron phase-relaxation length and to probe long-range adsorbate interactions.