We report inter-plane ($R_{zz}$) electrical transport measurements in the tp series of organic conductors at very high magnetic fields. In the field range between 36 and 60 T $R_{zz}$ shows a very hysteretic first order phase transition from metallic to an insulating state. This transition does not affect the Shubnikov-de-Haas oscillations associated with the two-dimensional (2D) Fermi surface. We argue that this transition originates from inter-plane disorder which gives rise to incoherent transport along the least conducting axis. We conclude that this system becomes a strictly 2D Fermi-liquid at high magnetic fields.
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