Weak scattering sources in YBa$_{2}$Cu$_{3}$O$_{6.5}$

We have measured the microwave conductivity of ultra-high purity single crystal YBa$_{2}$Cu$_{3}$O$_{6.5}$ as a continuous function of frequency (0.5 to 20 GHz) and at low temperatures. The samples were prepared in the highly ordered ortho-II phase, in which the off-plane oxygen atoms are arranged into alternating full and empty chains. Measurements were taken in this ordered state and then, following heating and rapid quench cooling, again in a disordered state. Our results help to illuminate the roles of both cation impurities and of CuO chain oxygen disorder as sources of in-plane quasi-particle scattering in ultra-high purity YBCO.