On the 2.8 V6 engines in the A4 and A6, the cylinder head breather hoses that feed oil vapor into the throttle boot are made of a very thin walled plastic, which over time becomes incredibly brittle. It is all too easy to break these hoses in to many little tiny pieces. Think egg shells here.
Add to this the fact that the replacement pieces from Audi are frightfully expensive. Last I checked, the piece that runs from the passenger side valve cover over to the driver's side is over $90.
I have struggled over the years to find a way to fix this. At first, I used plastic conduit to repair the broken pieces, but I never could get it to seal properly.
I've heard of people using heater hose to replace these sections, so I gave it a try. I chose 3/4 (19mm) id hose, it fits over the largest part of the original connectors on the plastic hose.
To install the new hose, remove the old plastic hose, and cut the connectors off, right where the 'ribbed' part of the hose begins. Clip the connector back onto it's home, and slide the hose over it, till it makes a decent seal. I used cable ties to secure the hose, you might want either hose clamps, or something. I found that the cable tie holds it fine, but required a fair bit of tension.
Run it in the same way as the original. You can flatten it a little by the coil pack. Re-mounting the engine cover is possible, but you have to press it down a little to flatten the hose. I suspect that 17mm id hose would be a better fit in this area, and it will fit on the smaller part of the connectors.
You can see that it's quite thick hose. Once the engine cover is back on, you can't really see it though.
With the zip ties only, I have not seen ANY leaking at all. The engine runs way better than before, I suspect that I was having severe vacuum leaks from my previous attempts at repairing these hoses.
The crud in the below image was there from before. The above image is after a few weeks of use.
Hope this helps save you some money, and fix your car. I used a 4 foot section, and had a bit left over.