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Kitaev quantum spin liquid (QSL) harbors exotic excitations such as Majorana fermions and non-Abelian anyons as a result of the fractionalization of quantum spins. One of the prime candidates for Kitaev magnets is a spin-orbit assisted Mott insulator α-RuCl₃ with layered honeycomb structure. In a field induced quantum disordered (FIQD) state of α-RuCl₃, half-integer quantized thermal Hall (HIQTH) effect, a signature of the chiral Majorana edge currents and non-Abelian anyons, has been reported. However, despite intensive research efforts, the exact nature of the FIQD state has been highly controvertial. For example, instead of the HIQTH effect, the longitudinal thermal conductivity κ_xx shows oscillatory feasures as a function of in-plane magnetic field both in zigzag antiferromagnetic and FIQD states, suggesting further exotic quantum spin states. Moreover, the HIQTH effect is observed only in the crystals with larger κ_xx in zero field below T_N, depending on growth method. These results call for more systematic studies using high-quality crystals grown by the different methods. Here we study the thermal transport properties in the ultra-clean α-RuCl₃ crystals grown by the sublimation method. In zero field, the magnitude of thermal conductivity peak below T_N is the largest reported so far, supporting the high quality of the crystals. In the field dependence of κ_xx, oscillatory features are not clearly observed, in contrast to expectation for the conventional quantum oscillations. We will also discuss the results of κ_xy in the ultra-clean crystals for the nature of the HIQTH effect in α-RuCl₃.