indicates that the charge/unit length in a tapered channel decreases as we move from source to drain. Yet, in the course of the integration leading to Eq. (3.12), it was argued that the current iD...

indicates that the charge/unit length in a tapered channel decreases as we move from source to drain. Yet, in the course of the integration leading to Eq. (3.12), it was argued that the current iD must be constant throughout the entire channel. How can a constant current exist in spite of a decreasing charge/unit length? Do you sense any contradiction here? Explain!