I have previously pointed out that the quality of German armour degraded sharply over a relatively short amount of time. But when did it start? Early war vehicles exhibit excellent armour ductility, but the late war ones, not so much. Since it's impossible to grab samples of the produced armour, I'm going to have to rely on second-hand information. Thankfully, a rather reputable figure speaks on the subject!
"In November of 1941, high ranking engineers, industry representatives, and armament directorate officers came to my tank army in order to familiarize themselves with the Russian T-34 tank. Frontline officers suggested that we should build tanks exactly like the T-34 in order to correct the unpleasant position of our armoured forces, but this position did not receive support from the engineers. Not because they were opposed to imitation, but because it was not possible to rapidly set up manufacturing of important components, especially the diesel motor. Additionally, our hardened steel, whose quality was dropping due to a lack of natural resources, was inferior to the Russians' hardened steel."
H. Guderian, "Panzer Leader", page 268
Interestingly enough, Daimler-Benz decided to imitate the T-34 anyway, with domestic components. The result proved complicated and unreliable, and lost out to the MAN prototype in competition for the Panther project.