  The microwave conductivity of an ultra-pure single crystal of
YBa$_2$Cu$_3$O$_{6.50}$ has been measured deep in the superconducting state as
a continuous function of frequency from $0.5\to20$ GHz. Conductivity spectra
were first measured at four temperatures below 10 K after having prepared the
crystal in the so called ortho-II phase in which the CuO chain oxygen are
ordered into alternating full and empty chains. These spectra exhibit features
expected for quasiparticle scattering from dilute weak impurities (small
scattering phaseshift) in an otherwise clean d-wave superconductor. The
measurements were repeated on the same crystal after heating and then rapidly
quenching the sample to reduce the degree of oxygen order in the CuO chains.
With the increased disorder, the conductivity spectra retain the distinctive
weak-limit scattering features, but have increased widths reflecting an
increase in quasiparticle scattering. These measurements unambiguously
establish that CuO chain oxygen disorder is the dominant source of in-plane
quasiparticle scattering in high purity YBCO.
