  We have used a combination of neutron resonant spin-echo and triple-axis
spectroscopies to determine the energy, fine structure, and linewidth of the
magnon resonance in the model spin-1/2 ladder antiferromagnet IPA-CuCl_3 at
temperatures T << Delta_0 /k_B, where Delta_0 is the spin gap at T=0. In this
low-temperature regime we find that the results deviate substantially from the
predictions of the non-linear sigma model proposed as a description of magnon
excitations in one-dimensional quantum magnets and attribute these deviations
to real-space and spin-space anisotropies in the spin Hamiltonian as well as
scattering of magnon excitations from a dilute density of impurities. These
effects are generic to experimental realizations of one-dimensional quantum
magnets.
