Sell-side banking analysts are not convinced about the big banks' reliance on income from volatile markets' business and the latter's ability to produce income growth in the March half-year.UBS analyst Jonathon Mott said in his commentary on the big banks' first half results: "We had concerns around the significant first half contribution from the trading and balance sheet elements of ANZ's global markets business. Ex-global markets, the momentum looked less impressive."Mott described this revenue source as volatile."In NAB's case, excluding a big swing in markets and treasury related income, revenue was down and core earnings flat," he said.UBS noted that Westpac's result was also boosted by treasury and trading income.Citi analyst Craig Williams also focused on the contributions of markets' income to overall results. UBS is expecting a pick-up in revenue for all the major banks in the second half. The average for the first half was 1.1 per cent, while the expectation for the full year is four per cent growth.It is expecting margin contraction to continue at about the same rate as during the first half. The big banks' margin contracted by an average of six per cent in the first half and UBS expects the full year number to also be six per cent.Citi's expectation is that net interest margins will rebound in the second half. UBS cut its ANZ recommendation from Neutral to Sell. It upgraded its Commonwealth Bank recommendation from Neutral to Buy after the bank's strategy update in April. It downgraded Westpac from Buy to Neutral and maintained a Neutral recommendation for NAB.Citi said CBA was its pick of the four big bank stocks, although it maintains a Neutral recommendation. It said: "CBA has been left behind in this year's rally in bank prices. "Ex-markets income, it has recorded the strongest revenue growth in the last three reporting periods and its domestic focus generates superior returns."Citi has a Neutral recommendation on all four stocks.