Here the depth of the well is less than before and the width of the well is just half the length of the cell. The potential now is weak enough that the energies, except near the first gap, are all close to the free electron values. For most choices of energy the wave functions also are the corkscrews (spiral staircases) characteristic of the plane wave psi(x) = exp(i*k*z).
In the "Nearly Free Electron" approximation, the energy gaps are twice the value of the appropriate Fourier coefficient in the Fourier expansion of the periodic potential. For the square well potential, the amplitudes of these Fourier components for the odd numbered gaps fall off as (1/n) where n is the gap index. What is the ratio of the sizes of the first and third gaps? (Click on the "band list" toggle to get a listing of the energies of the band edges.)
Why is the second gap smaller than both the first and third? Why is it not precisely zero?