Limiting sensor input voltage without biasing measurement
Zeners have a very 'soft' characteristic. As you discovered, they start drawing current well before their advertised voltage.

The best way to limit overvoltage into an ADC is to use a silicon diode to a fixed voltage. Usually this will be the ADC rail, especially if you put a series resistor between the diode and the ADC to limit any current drawn, but you could use a biassed zener to provide the fixed voltage if the rail is not suitable.

While general purpose diodes like 1N4148 are fairly low leakage, there are specialist 'low leak' diodes like BAS116. Here are some figures I took a while ago, to characterise diodes used for ADC protection.

Reverse leakage, and effective resistance around 0v at 15°C.   

diode    reverse leakage   slope resistance over +/- 10mV
-----    ---------------   ------------------------------
1N4148      4nA  (5V)            30Mohm
BAT42      35nA  (5V)             1Mohm
BAS116    &lt;10pA  (30V)         &gt;&gt;20Gohm (10pA was measurement resolution)


The BAS116 reverse leakage typ/max spec at 25°C is 3pA/5nA, and 3nA/80nA at 150°C

Here's the forward conduction current of BAS116 versus 1N4148
(it might have been a quite leaky 4148 sample, below 300mV it conducted more than a 1N540x)

 Vf     BAS116   1N4148
----    ------   ------
300mV    40pA     1.3uA
450mV    45nA     27uA
640mV    16uA     1.7mA