  We analyze quantum interference and decoherence effects in single-molecule
junctions both experimentally and theoretically by means of the mechanically
controlled break junction technique and density-functional theory. We consider
the case where interference is provided by overlapping quasi-degenerate states.
Decoherence mechanisms arising from the electronic-vibrational coupling
strongly affect the electrical current flowing through a single-molecule
contact and can be controlled by temperature variation. Our findings underline
the all-important relevance of vibrations for understanding charge transport
through molecular junctions.
