Phase coherence lengthL&fgr;of on‐facet quantum wires is determined at temperatures down to 50 mK by three methods: magnetoresistance, conductance fluctuation amplitude, and the conductance fluctuation correlation field. It is found thatL&fgr;saturates below about 0.4 K. This saturation is identified as due to spin‐orbit interaction, which has no temperature dependence. This result is supported by positive magnetoresistance under a weak magnetic field.