  We present a study of the magnetic field-dependent thermal transport in the
spin S=1 chain material Ni(C2H8N2)2NO2(ClO4) (NENP). The measured thermal
conductivity is found to be very sensitive to the field-induced changes in the
spin excitation spectrum. The magnetic contribution to the total heat
conductivity is analyzed in terms of a quasiparticle model, and we obtain a
temperature and momentum independent mean free path. This implies that the
motion of quasiparticles is effectively three dimensional despite the tiny
interchain coupling.
