  A finite residual linear term in the thermal conductivity at zero temperature
in insulating magnets indicates the presence of gapless excitations of
itinerant quasiparticles, which has been observed in some candidate materials
of quantum spin liquids (QSLs). In the organic triangular insulator
beta'-EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature
thermal conductivity depends on the cooling process and the finite residual
term is observed only in samples with large thermal conductivity. Moreover, the
cooling rate dependence is largely sample dependent. Here we find that, while
the low-temperature thermal conductivity significantly depends on the cooling
rate, the high-temperature resistivity is almost perfectly independent of the
cooling rate. These results indicate that in the samples with the finite
residual term, the mean free path of the quasiparticles that carry the heat at
low temperatures is governed by disorders, whose characteristic length scale of
the distribution is much longer than the electron mean free path that
determines the high-temperature resistivity. This explains why recent X-ray
diffraction and nuclear magnetic resonance measurements show no cooling rate
dependence. Naturally, these measurements are unsuitable for detecting
disorders of the length scale relevant for the thermal conductivity, just as
they cannot determine the residual resistivity of metals. Present results
indicate that very careful experiments are needed when discussing itinerant
spin excitations in beta'-EtMe3Sb[Pd(dmit)2]2.
