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To experimentally check the predictions of the thermal model for the influence of thermal conductivity on breakdown field levels in Nb cavities, eight elliptical cavities at 8600 MHz have been built from material with RRRs of 25, 50 and 100 (RRR α thermal conductivity). After several rf tests on each cavity, the RRRs were increased by outgassing at -2000 C in a vacuum of torr using resistive and induction heating furnaces with high pumping speeds. From a total of 40 tests on these cavities with RRRs ranging from 25 to 1400 the breakdown field is found to be roughly proportional to the square root of the RRR. Fabrication methods were kept the same for all the cavities and identical chemical surface preparation procedures were applied to the cavities before and after changing RRRs.