  Magnetotransport measurements are performed in ultraclean (lithographically
patterned) graphene nanoribbons down to 70 nm. At high magnetic fields, a
fragmentation of the electronic spectrum into a Landau levels pattern with
unusual features is unveiled. The singular Landau spectrum reveals large
magneto-oscillations of the Fermi energy and valley degeneracy lifting. Quantum
simulations suggest some disorder threshold at the origin of mixing between
opposite chiral magnetic edge states and disappearance of quantum Hall effect.
