We report experimental evidence of the first-order phase transitions in the two-dimensional electron gas formed in a gated wide GaAs/AlGaAs quantum well at even-integer quantum-Hall states, At the filling factor values of v = 2, 4 and low temperatures, crossing of Landau levels through the application of the gate bias yields a suppression of the quantum-Wall-state excitation gap and hysteretical behaviour of the diagonal resistivity in up and down sweeps of the magnetic field. Detailed many-body calculations indicate the occurrence of a first-order phase transition and allow the determination of the exact properties of the electron ground states involved in the transition. (C) 2000 Published by Elsevier Science B.V. All rights reserved.