A refrigerant unit associated with a product transport container is provided with a dual-path, parallel flow expansion circuit. The expansion circuit includes a primary expansion device disposed in a primary refrigerant flow path and an auxiliary expansion device disposed in a secondary refrigerant flow path. During operation of the refrigeration unit in a stable temperature maintenance mode, refrigerant flow is supplied to the evaporator coil through the primary refrigerant flow path only. During operation of the refrigeration unit in a temperature pull-down mode, to increase the refrigerant mass flow through the evaporator coil, refrigeration flow is supplied to the evaporator coil through the primary refrigerant flow path and the secondary refrigerant flow path of the expansion circuit.