It's all because of the fact that the energy levels are discrete, quantized levels, with gaps in between them.

都是因为能级是分立的事实，量子化的能级，相互之间有间隔。

youdao

Thus, confinement effects modify electron energy levels, similar to a particle in a box, whereby Coulomb blockade effect arises as a consequence of the discrete nature of electric charges.

因此，限制效应改变了电子能水平，就好像一个装在盒子中的颗粒，这就是为什么电荷的离散特性会导致库仑阻塞效应的原因。

youdao

Electrons in a cool material are confined to a restricted number of discrete energy levels, with the lowest occupied first.

ECONOMIST: Microrefrigeration