= Solution
A larger source–drain bias opens an <energy> window between reservoir <electrochemical potentials>. When a charge-addition/removal transition enters that window, sequential tunnelling becomes allowed even away from a zero-bias degeneracy. The low-conductance regions in a gate/bias map are \b[<Coulomb diamonds>].
For an illustrative symmetric device with reservoir potentials $\pm eV_{\rm SD}/2$, let $q=Q_{\rm ext}/e$ and $E_{\rm ch}=e^2/C$. In the charge-$N$ interval, the classical blockade domain is
$$
|eV_{\rm SD}|<E_{\rm ch}[1-2|q-N|].
$$
It shrinks to zero at $q=N\pm1/2$. Unequal <capacitances> change edge slopes and voltage division; the figure's symmetry is a chosen model, not a universal device property.
Quantum confinement adds discrete single-particle levels and spin structure. In a constant-interaction approximation, successive addition energies contain $e^2/C$ plus the relevant orbital spacing, so diamond sizes need not all match. Excited-state tunnelling can produce extra conductance lines. At stronger coupling, <cotunnelling> and charge fluctuations can give leakage beyond a simple sequential classical model. Thus <temperature> and coupling must be small enough to resolve the charging and level structure separately.
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