[en] In this work we investigate quantum transport in Cu nanowires created by bringing macroscopic Cu wires into and out of contact under an applied magnetic field in air. Here we show that a 70% magneto-conductance effect can be seen in a Cu nanowire in a field of 2 mT at room temperature. We propose that this phenomenon is a consequence of spin filtering due to the adsorption of atmospheric oxygen modifying the electronic band structure and introducing spin-split conduction channels. This is a remarkable result since bulk Cu is not magnetic and it may provide a new perspective in the quest for spintronic devices. (letter to the editor)