  We report experimental evidence for atomic chain formation during stretching
of atomic-sized contacts for gold and silver, that is strongly enhanced due to
oxygen incorporation. While gold has been known for its tendency to form atomic
chains, for silver this is only observed in the presence of oxygen. With oxygen
the silver chains are as long as those for gold, but the conductance drops with
chain length to about 0.1 conductance quantum. A relation is suggested with
previous work on surface reconstructions for silver (110) surfaces after
chemisorption of oxygen.
