What is the current state of the law on consultation between expert and counsel?

Consultation between expert and counsel may appropriately be given broad immunity from discovery, both as to expert and as to counsel, because none of the expert's opinion, professional though it may be, is relevant evidence in the case. To the contrary, his opinion is and will remain wholly irrelevant and immaterial as evidence until the expert is called as a witness on the trial and shown to be qualified to give competent opinion testimony on a matter in which he is versed and which is material to the case. (United States v. Certain Parcels of Land, D.C., 15 F.R.D. 224, 233.)