If the stops on a rear brake cable run have slots it's possible to demount the cable for inspection, cleaning and lubrication, without unclamping the inner-cable.
Open The Brake
Open the brake or if there is a quick-release on the lever open that.
Dismount the rear brake cable
The rear brake cable will now be slack so it can be released from the slotted cable guides and the inner-cable can be inspected and cleaned and lubricated if necessary.