Maniscalco, J.T. Ge, M. Gonnella, D. Liepe, M. JACoW Geneva, Switzerland 978-3-95450-180-9 10.18429/JACoW-NAPAC2016-MOPOB65 English 218-220 MOPOB65 ion cavity ECR experiment SRF Contribution to a conference proceedings 2017 2017-01 http://dx.doi.org/10.18429/JACoW-NAPAC2016-MOPOB65 https://jacow.org/napac2016/papers/mopob65.pdf The surface resistance of a superconductor, a property very relevant to SRF accelerators, has long been known to depend on the strength of the surface magnetic field. A recent discovery showed that, for certain surface treatments, microwave cavities can be shown to have an inverse field dependence, dubbed the ‘‘anti-Q-slope'', in which the surface resistance decreases over an increasing field. Here we present an investigation into what causes the anti-Q-slope in nitrogen-doped niobium cavities, drawing a direct connection between the electron mean free path of the SRF material and the magnitude of the anti-Q-slope. Further, we incorporate residual resistance due to flux trapping to calculate an optimal mean free path for a given trapped flux.