We report on (magneto)transport experiments in chemically derived narrow graphene nanoribbons under high magnetic fields (up to 60 T). Evidences of field-dependent electronic confinement are given and allow estimating the possible ribbon edge symmetry. A large positive magnetoconductance indicates a strong suppression of backscattering induced by the magnetic field. Such scenario is supported by tight-binding calculations which consider different types of underlying disorders (smooth edge disorder and long-range Coulomb scatterers).